1. Field of the Invention
The invention relates to a highly machinable, prehardened, martensitic steel article used for metal die casting die components and other hot work tooling components, and to a method for producing the same.
2. Discussion of the Related Art
The typical method of manufacture of die components used for die casting, including light metals such as aluminum, and for other types of hot work tooling components consists of rough machining the component close to finish dimensions from a hot work tool steel die block, hardening the rough-machined component by a quenching and tempering type of heat treatment, and finally machining the hardened component to finish dimensions. The performance and longevity of die components so manufactured are significantly affected by two features of this manufacturing procedure, namely, the quenching rate employed to harden the component.sup.1/2/ and the technique used to finish machine the component..sup.3/ For AISI hot work tool steels, rapid quenching rates are required to produce the martensitic microstructure necessary for long service life. Slow quenching rates minimize size change and distortion of the rough-machined component, and thereby reduce the amount, severity, and cost of the finish machining operation. The slow quenching rates, however, also reduce service life, because they introduce nonmartensitic constituents into the microstructure of the steel. The size change and distortion of quenched, rough-machined die components can be eliminated while maintaining the optimum, rapidly-quenched, martensitic microstructure by manufacturing the die components from prehardened hot work tool steel die blocks. FNT .sup.1/ Cocks, D. L., "Longer Die Life from H13 Die Casting Dies by the Practical Application of Recent Research Results," Die Casting Research Foundation (now the North American Die Casting Association), Techdata Digest No. 01-88-01D, Apr., 1988. FNT .sup.2/ Wallace, J. F., et al., "Influence of Cooling Rate on the Microstructure and Toughness of Premium H-13 Die Steels," Transactions Of the North American Die Casting Association 15th International Congress, Oct. 16.noteq.19, 1989, Paper G-T89-013. FNT .sup.3/ Dorsch, C. J. and Nichols, H. P., "The Effect of EDM on the Surface of Hardened H-13 Die Components," Transactions of the North American Die Casting Association 15th International Die Casting Congress, Oct. 16-19, 1989, Paper G-T89-031.
Prehardened die blocks made from conventional, resulfurized AISI H13 hot work tool steel are currently available. The sulfur additions in the steel make it machinable at the high hardness needed for die casting applications (35 to 50 HRC), but die components manufactured from the currently available prehardened die blocks exhibit short service life because the sulfur in the steel reduces thermal fatigue resistance and impact toughness, which in turn reduce die performance and die service life. .sup.4/ FIGS. 1 and 2 are excerpted from this reference.sup.4/ and show the detrimental effect of higher sulfur content on the thermal fatigue resistance of AISI H13 hot work tool steel. Similarly, FIG. 3 is also from this reference and shows the detrimental effect of increasing sulfur content on the dynamic fracture toughness of AISI H13. This reference concludes that: "Higher sulfur levels of the H-13 steels above 0.028% reduce thermal fatigue resistance. The fracture toughness of H-13 steel hardened for use in die casting dies is reduced steadily by raising the sulfur content of the steel from 0.003 to 0.008 to 0.014 to the 0.028-0.075%S range. This behavior is attributed to the effect of the inclusions produced by higher sulfur levels." In response to the results of the work in the referenced literature, and because of the significant economic impact which results from reduced thermal fatigue resistance in die casting dies, the North American Die Casting Association has limited the sulfur content of AISI H13 which is considered to be of premium quality for die casting die applications to a maximum of 0.005 weight per cent. FNT .sup.4/ Pixi Du and J. F. Wallace, "The Effects of Sulfur on the Performance of H-13 Steel," Die Casting Research Foundation (now the North America Die Casting Association), Techdata Digest Number 01-83-01D, 1983.
The potential industry wide cost savings which could result from the use of highly machinable, prehardened die blocks is offset by the reduction in die component life which is inherent in the currently available prehardened die blocks. A need therefore exists for a highly machinable, prehardened, martensitic hot work tool steel die block that can be used without sacrificing die performance and longevity.